System for intervertebral disk prostheses

ABSTRACT

System of intervertebral disc prostheses which includes standard prostheses and corrective prostheses. The corrective prostheses serve to compensate for a ventro-dorsal offset. They are characterized in that, on one of their cover plates ( 15 ), the surface ( 4 ′) via which it cooperates in a matching manner with the prosthesis core ( 3 ′) is offset ventrodorsally relative to the contact surface ( 7 ) by comparison with the standard prostheses.

[0001] It is known to replace damaged intervertebral discs withprostheses which consist of two cover plates, each to be connected to anadjacent vertebral body, and of a prosthesis core, said prosthesis corecooperating with one or both cover plates via complementary sphericalsurfaces of articulation (EP-B 298 233). The cover plates have contactsurfaces by means of which they are connected to the adjacent vertebralbodies. It is desired that the centre of articulation of the prosthesesbe arranged in such a way that the movements permitted by the prosthesisare as far as possible identical to the natural ones and that forces canbe transmitted uniformly between the vertebral bodies and theprosthesis. In known prostheses, this objective is approached byarranging the centre of articulation in a predetermined spatialrelationship to the contact surfaces of the cover plates and byproviding the cover plates with an edge which bears on the ventralmargin of the associated vertebral body and thereby determines therelative position of the cover plate to the vertebral body (EP-B 560140), or by using an implantation instrument which has a limit stop onthe vertebral body (EP-B 333 990). This ensures an at all timesidentical position of the centre of articulation of the prosthesis inrelation to the ventral edge of the vertebral bodies. It is also known(EP-A 955 021), in a system of intervertebral disc prostheses includingseveral categories of different sizes, to provide corrective prostheseswhich belong on one side to one size category and on the other side toanother size category.

[0002] By means of the abovementioned predetermined spatial relationshipof the centre of articulation to the contact surfaces of the prostheses,the desired spatial relationship to the vertebral bodies is achievedonly if the anatomical conditions correspond to the normal conditionsassumed upon construction of the prosthesis. If, however, for examplefor anatomical reasons, a centre of articulation is by way of exceptionto have another position, the prosthesis in question or a cover plate ofthis prosthesis must be implanted so as to deviate from the normalspatial relationship to the vertebral body, which is difficult andrisky.

[0003] Starting from the prior art last mentioned above, it is thereforean object of the invention to make the implantation of the prosthesiseasier and safer for these cases. This is achieved by the features ofclaim 1 and preferably by the features of the subclaims.

[0004] A system of intervertebral disc prostheses is assumed whichincludes standard prostheses and corrective prostheses. The standardprostheses in each size category are identical to each other. Aplurality of size categories are normally available, although this isnot absolutely necessary. The prostheses consist of a prosthesis coreand of at least one cover plate. The prosthesis core cooperates with atleast one cover plate via articulation surfaces. The prosthesis corepreferably has two articulation surfaces on opposite sides via which itcooperates with complementary articulation surfaces of two cover plates.However, it is also possible for one of the cover plates to cooperate ina non-articulating manner with the prosthesis core via retainingsurfaces. Articulation surfaces and retaining surfaces are combinedbelow and in the claims under the term core-matching surfaces. The coverplates have a contact surface to adjoin a vertebral body. In thisconnection, the invention provides for the corrective prostheses to haveat least one corrective cover plate whose core-matching surface isoffset ventrodorsally relative to the contact surface by comparison withthe standard prostheses.

[0005] In this way, it is ensured that the centre of articulation of aprosthesis relative to the vertebral body on whose side the correctivecover plate is fitted is offset ventrodorsally compared to the standardprostheses. The offset can be in the ventral direction or the dorsaldirection depending on the type and direction of insertion of the coverplate. If the contact surfaces of the corrective cover plates are ofsymmetrical configuration in relation to their mediolateral centre line,they can either be used with the centre offset in the ventral directionor in the dorsal direction. Since the offset in question ispredominantly in the dorsal direction, an essential embodiment of thecorrective rective plate according to the invention is distinguished bythe fact that the centre of articulation is offset dorsally, providedthat the dorsal side can be distinguished on the cover plate.

[0006] Cases arise in which the intervertebral disc prosthesis tends tobe offset in an undesired manner because of considerable curvature ofthe spinal column or because of high ventrodorsal forces acting betweenthe vertebrae concerned. This is true in particular of those prosthesesin which the prosthesis core cooperates with both cover plates viaspherical articulation surfaces. In this type of prosthesis, the coverplates are able to move slightly in translation relative to each otherin an angular position of the prosthesis core under relative lateralforces. The use of the invention is particularly advantageous in thistype of prosthesis because the corrective plates make it possible toarrange the centre of articulation of one cover plate so that it isdisplaced relative to the centre of articulation of the other coverplate, as a result of which said offset is compensated.

[0007] The diameter of the articulation surfaces can be chosen as largeas possible within the limits of the cover plate in order to keep thesurface pressure low. In known prostheses, the articulation surfaceformed on the cover surface has an only slightly smaller ventrodorsalextent than the cover plate. In these cases the offset of thearticulation surface relative to the contact surface can take place onlyunder the proviso that a smaller diameter is chosen for the articulationsurface. If the radius of curvature of the articulation surface is leftunchanged, the depth of the articulation surface provided in the coverplate decreases as the diameter decreases and thus its ability totransmit forces acting laterally (parallel to the plane of the coverplate). This may be acceptable in some cases, so that this embodiment ofthe invention is not excluded. However, an embodiment is preferred inwhich the diameter and the radius of curvature of the articulationsurface of the corrective cover plate are chosen to complement thearticulation surfaces which belong to another, smaller size category. Inthe smaller size category, not only is the diameter of the articulationsurfaces chosen smaller, but also the radius of curvature. The depth ofthese smaller articulation surfaces is therefore greater than the depthof an articulation surface of the larger size category, in which onlythe diameter was reduced, but not the radius of curvature.

[0008] Choosing an articulation surface (or other core-matching surface)admittedly rules out using the corrective cover plate together withprosthesis parts which belong to the same size category. However,instead of this, prosthesis parts can be used which belong to thesmaller size category, on whose basis the articulation surface of thecorrective cover plate was chosen. The prosthesis core is preferably inall cases a standard prosthesis core. The second cover plate belongingto the prosthesis is likewise preferably a standard cover plate.However, if a particularly large offset of the two contact surfaces ofthe prostheses relative to each other is wanted, the second cover platecan also be a corrective cover plate, which is, however, rotated though180° in relation to the first one so that the offset amounts of bothplates are added to each other. It is only if, for some reason, thecentre of articulation of the prosthesis is to be displaced ventrally ordorsally in relation to both adjacent vertebrae that corrective coverplates are used in the same orientation on both sides.

[0009] In the case of the corrective cover plates, as a result of theoffset arrangement of the core-matching surface relative to the contactsurface, on one side of the core-matching surface there is a distancebetween this and the plate edge. If this part of the surface is notneeded in order to make available as large a contact surface aspossible, the plate can be shortened on this side. The contact surfaceis then the same width in the mediolateral direction as the standardcover plates of the same size category, but its dimension in theventrodorsal direction is smaller and can correspond to the smaller sizecategory. This can afford advantages in terms of adjustment in caseswhere the vertebral bodies are wider in the mediolateral direction andnarrower in the ventrodorsal direction.

[0010] The invention is explained in more detail below with reference tothe drawing which depicts advantageous illustrative embodiments and inwhich:

[0011]FIG. 1 shows a medial section through a standard prosthesis of afirst size category,

[0012]FIG. 2 shows a view of the inside of a cover plate of a standardprosthesis of the first size category,

[0013]FIG. 3 shows a medial section through a standard prosthesis of asecond size category,

[0014]FIG. 4 shows a view of the inside of a cover plate of the standardprosthesis according to FIG. 3,

[0015]FIG. 5 shows a first embodiment of a corrective prosthesis inmedial section,

[0016]FIG. 6 shows a view of the inside of the associated correctivecover plate,

[0017]FIG. 7 shows a second embodiment of the corrective prosthesis inmedial section,

[0018]FIGS. 8, 9 show medial sections through standard prostheses ofdifferent size categories in a modified embodiment, and

[0019]FIGS. 10, 11 show two corrective prostheses for the modifiedembodiment.

[0020] The standard prostheses according to FIGS. 1 to 4 consist of alower cover plate 1, an upper cover plate 2 and a prosthesis core 3. Thecover plates form spherical, concave articulation surfaces 4, and theprosthesis core 3 forms two identical convex, spherical articulationsurfaces 5 which lie opposite each other and which are designed tocomplement those of the cover plates. All dimensions of the first sizecategory (FIGS. 1 and 2) are greater than those of the second sizecategory (FIGS. 3 and 4). In addition to the two size categories shown,further size categories can be present within the system.

[0021] The components of the prosthesis can be made of materials whichhave proven suitable for endoprostheses, for example metal, ceramic,polyethylene, with the cover plates 1, 2 preferably being made of rigidmaterial (for example metal) and the prosthesis core preferably beingmade of polyethylene.

[0022] The cover plates 1, 2 have a contact surface 7 which is intendedfor connection to the end face of a vertebral body. It can be equippedwith means (not shown) for fixed connection to the bone, for exampleteeth. It is more extensive in the mediolateral direction, which isindicated by the line 10 in FIG. 2, than it is in the ventrodorsaldirection 11. The diameter 12 of the. articulation surface 4 in thestandard prostheses is as large as is permitted by the dimension of thecover plate in the direction 11. These explanations concerning FIGS. 1and 2 apply likewise to the second size category according to FIGS. 3and 4.

[0023] As is known, such prostheses are able to transmit from vertebrato vertebra the forces extending in the direction of the spinal column,and to a certain extent also the forces extending transverse to thelatter, and at the same time they permit swivel movements. If they areimplanted between vertebrae between which strong transverse forces act,or between which a substantial directional change takes place, as isoften the case for example between the last lumbar vertebra and thesacrum, a displacement of the cover plates and of the vertebrae mayoccur. This can be compensated for by using a corrective prosthesis 14.It is constructed, for example, as is shown in FIG. 5 or FIG. 7.

[0024] According to FIG. 5, the corrective prosthesis has an upper coverplate 15 which is designed as a corrective cover plate. The externaldimensions of its contact surface 7 and thus the overall surfacedimensions are equal to those of the standard cover plate 2 of the firstsize category. The centre 16 of the articulation surface 4′ provided onits inside is displaced dorsally in relation to the centre line 10 by anamount 17. If one were to use the articulation surface 4 of the standardcover plate for this, it would assume the position 4″ indicated by thebroken line in FIG. 2. As this is partially extending out beyond theedge of the standard prosthesis, the latter would have to be provided atthis location with a projection or a widening, or the articulationsurface would have to be shortened to correspond to the standard edgecontour. Both of these options are within the scope of the invention butare not generally expedient. The embodiment according to FIG. 6 is thuspreferred, in which use is made of that articulation surface 4′ of thesecond size category whose diameter 12′ is correspondingly smaller andcan therefore be accommodated within the edge of the standard format ofthe corrective cover plate 15. Matching this articulation surface 4′,the corrective cover plate 15 is combined with a prosthesis core 3′ anda lower cover plate 1′ of the second size category, as is indicated inFIG. 5. If, in this corrective prosthesis, the position of thearticulation centre line 16 is compared with the position of the contactsurface 7 of the corrective cover plate 15, it can be seen that, unlikein the standard prosthesis, these do not coincide but instead are offsetin relation to one another by the amount 17.

[0025] Instead of being connected to a standard cover plate 1′ of thesecond size category (FIG. 5), a corrective cover plate 15 turnedthrough 180° can also be used as lower cover plate in the correctiveprosthesis (FIG. 7). The offset 17 obtained on the lower cover plate isadded to the offset 17 on the upper cover plate with the result that thecorrective prosthesis according to FIG. 7 provides twice the offset ofthat according to FIG. 5.

[0026] As has been stated, the invention is especially suitable for theprosthesis type shown in FIGS. 1 to 7 in which the prosthesis core 3 hastwo articulation surfaces 5 opposite each other. However, the inventioncan also be used in prostheses of the prosthesis type shown in FIGS. 8to 11. FIGS. 8 and 9 show medial sections through prostheses ofdifferent size categories. They consist of a lower cover plate 21, 21′,an upper cover plate 22, 22′ and a prosthesis core 23, 23′. The uppercover plate 22, 22′ and the top of the prosthesis core 23, 23′ areidentical to those of the illustrative embodiment discussed above. Theyafford a possibility of articulated movement along the pair ofarticulation surfaces 24, 25. The underside of the prosthesis core 23 ismade flat. The lower cover plate 21, 21′ and its core-matching surface26 with edge 27 is designed solely to hold the prosthesis core 23, 23′.

[0027] According to FIG. 10, the upper cover plate 20 is designed as acorrective cover plate according to the explanations given for FIGS. 5and 6. It belongs to the first, larger size category. It is combinedwith a prosthesis core 23′ and a lower cover plate 21′ of the second,smaller size category.

[0028] Instead of this, it is also possible according to FIG. 11 to usethe lower cover plate 29 as corrective plate. It belongs to the firstsize category and is combined with a prosthesis core 23′ and an uppercover plate 22′ of the second size category. As in the examplesdiscussed above, this therefore results in an offset 17 of the centre ofarticulation relative to the contact surface of the corrective coverplate.

[0029] For the sake of simplicity, the offset mentioned in theexplanations has been described relative to the contact surface's centrepoint which, in the standard prostheses, coincides with the centre ofarticulation of the prosthesis. It goes without saying, however, thatthe offset can be determined relative to any desired point of thecontact surface, in which case its relative position to the articulationcentre is to be compared, on the one hand, in a standard cover plateand, on the other hand, upon use of a corrective cover plate.

[0030] As can be seen in FIG. 6, the edge strip 30 of the correctivecover plate 15 is not needed for accommodating the articulation surface4′. If it is also not needed with respect to a desired size of thecontact surface 7, it can be dispensed with. The cover plate 15 and itscontact surface 7 are then delimited for example according to the brokenline 31.

1. System of intervertebral disc prostheses which includes standardprostheses and corrective prostheses, the standard prostheses in atleast one size category being identical to each other and consisting ofa prosthesis core (3, 3′, 23, 23′) and of at least one cover plate (1,1′, 2, 2′, 21, 21′, 22, 22′) which has a core-matching surface (4, 4′,24, 26) cooperating with the prosthesis core (3, 3′, 23, 23′) and acontact surface (7) intended to adjoin a vertebral body (9),characterized in that the corrective prostheses (14) have a correctivecover plate (15, 20, 29) whose core-matching surface (4′, 28, 26) isoffset ventrodorsally relative to the contact surface (7) by comparisonwith the standard prostheses.
 2. System according to claim 1,characterized in that a dorsal side is distinguishable on the correctivecover plates (15, 20, 29), and the core-matching surface (4′, 26, 28) isoffset dorsally relative to the contact surface (7).
 3. System accordingto claim 1 or 2, characterized in that the diameter (12′) of thecore-matching surface (4′, 26, 28) of the corrective cover plates (15,20, 29) is smaller than that of the standard cover plates.
 4. Systemaccording to claim 3, characterized in that it includes a first sizecategory with standard cover plates (1, 2, 21, 22) whose core-matchingsurfaces (4, 24) have a first diameter (12), and a second size categorywith standard cover plates (1′, 2′, 21′, 22′) whose core-matchingsurfaces (4′, 24′, 26) have a second diameter (12′) which is smallerthan the first diameter (12), and in that the first size categoryincludes corrective cover plates (15, 20, 29) whose core-matchingsurface diameters (12′) are identical to the core-matching surfacediameter (12′) of the standard cover plates (1′, 2′, 21′, 22′) of thesecond size category.
 5. System according to one of claims 1 to 4,characterized in that it includes corrective prostheses which consist oftwo corrective cover plates (15), arranged in opposite directionsrelative to each other, and of a prosthesis core (3′).
 6. Systemaccording to one of claims 1 to 5, characterized in that thecore-matching surfaces (4, 24, 4′, 24′) are spherical articulationsurfaces.
 7. System according to one of claims 4 to 6, characterized inthat it includes corrective cover plates (15) whose width in themediolateral direction (10) is equal to the width of the standard coverplates (1, 2, 21, 22) in the same size category and whose dimension inthe ventrodorsal direction (11) is smaller than that of the standardcover plates (1, 2, 21, 22) in the same size category.